Some seats had foam and other cotton padding
I just finished rebuilding the front seat in my 55 Ford convertible and it had foam in the cushion and cotton in the backrests,while rebuilding it I put in a spare seat that looks to be from a base model 56 Merc and I noticed it was worse then my worn out original seat. I initially thought it was probably worn out like the original but after I installed the rebuilt original seat in my 55 I noticed it had all cotton on the cushion so was foam new in the 50s and could the higher trim level cars got the foam while the base models had the probably cheaper cotton padding.
